Silicates allotropic forms

Silicon is a hard, gray, semiconducting solid that melts at 1410°C. It crystallizes in a diamondlike structure but does not form a graphitelike allotrope because of the relatively poor overlap of silicon it orbitals. In nature, silicon is generally found combined with oxygen in Si02 and in various silicate minerals. It is obtained in... 

Carbonates and reduced forms of carbon are common on Earth, and silicates make up the major part of the crust germanium is much less common. All elements can form the diamond structure graphite and other allotropes are unique to carbon. 

Silicon accounts for 27.7% of the mass of the Earth s crust and occurs in a wide variety of silicates with other metals, clays, micas, and sand, which is largely Si02. The element is obtained on a small scale by the reduction of silicon(IV) oxide (Si02) by carbon or calcium carbide. For semiconductor applications very pure silicon is produced by direct reaction of silicon with an HCI/CI2 mixture to give silicon tetrachloride (SiCb), which can be purified by distillation. This is then decomposed on a hot wire in an atmosphere of hydrogen. For ultra-pure samples zone refining is used. Unlike carbon, silicon does not form allotropes but has only the diamond type of structure. 

In the following sections, we look at several nonmetallic materials with applications in modem technology. We begin with a discussion of the different allotropes or forms of carbon—diamond, graphite, and the fullerenes—where research has produced some exciting discoveries (Section 13.4). The fullerenes are recently discovraed molecular forms of the element carbon, in which the carbon atoms form hollow balls and tubes that may make them important as catalysts or possibly as drug-delivay mataials. Diamond shows promise as a material that might supersede silicon in its role as a matmal for sohd-state electronics. Silicon and diamond can act as semiconductors, which we discuss in Section 13.5. We end the chapter with sections on silicon, silica, and silicates (Section 13.6), ceramics (Section 13.7), and finally composites (Section 13.8). 

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